1. Field of the Invention:
This invention relates to a coordinate input apparatus and method. More particularly, the invention relates to an apparatus and method for entering a coordinate position by detecting input oscillation and specifying the position of the oscillation source.
2. Description of the Related Art:
A method of entering coordinates utilizing ultrasonic oscillation is known in the art. In an apparatus that employs this coordinate input method, a material such as glass or metal is used as a tablet (an oscillation transmitting panel) that forms a coordinate input surface, and a plurality of oscillation sensors are arrayed on the tablet at prescribed positions.
Using an input pen having an oscillator, the operator designates a desired position on the tablet, whereby oscillation generated from the tip of the pen is caused to propagate along the tablet. Each oscillator senses this oscillation, and the time from application of oscillation to the tablet to sensing of this oscillation differs depending upon the distance between the input pen and each oscillation sensor. In other words, by measuring transmission delay time from emission of oscillation from the input pen to sensing of the oscillation by each oscillation sensor, the distance from each oscillation sensor to the location designated by the input pen is obtained, thus making it possible to obtain the coordinate position of the location designated by the input pen. The time from entry of oscillation to the sensing thereof is sometimes referred to simply as delay time.
This delay time from emission of oscillation from the input pen to sensing of the oscillation by each oscillation sensor is measured in order to determine the distance from each oscillation sensor to the location designated by the input pen. Strictly speaking, the transmission delay time measured includes the time needed for oscillation to propagate through the interior of the input pen and reach the tip of the pen. Since the delay time attributed to oscillation through the input pen is included equally in all of the transmission delay times associated with the plurality of oscillation sensors, at calculation of coordinates the former delay time is subtracted as an offset from the delay time associated with each sensor in order to determine the coordinates.
A problem which arises is that the transmission delay time of oscillation within the input pen varies owing to a change in the environment such as temperature, as a result of which the wrong coordinates are calculated. The reason for this is that the resin (selected to improve writing ease when the pen is used) used in the tip of the input pen transmits sound at a speed slower than that of a material such as glass used in the tablet. In addition, since a change in the speed of sound with temperature is conspicuous, the amount of change in oscillation delay time within the input pen caused by a change in temperature is much greater in comparison with that of the tablet.
Further, since the delay time until oscillation is sensed by each sensor is measured using the time at which oscillation is produced by the input pen as a reference point, it is required that oscillation of the input pen be produced in synchronism with the main body that senses this oscillation. Accordingly, even if the input pen is cordless, there is a need for separate cordless communication means for the purpose of establishing synchronism with the main body. This is a cause of higher cost.